An important type of memory for computer systems is non-volatile memory, i.e., memory which retains its data without bias to the circuit. Semiconductor memories which provide non-volatile capability include fusable link programmable read only memories (commonly called PROMs), which distinguish between open and closed fuses to determine the data state. Other non-volatile semiconductor memories include ultra-violet erasable programmable read only memories (commonly called EPROMs), and electrically erasable programmable read only memories (commonly called EEPROMs and EAROMs). The EPROM and EEPROM devices utilize floating-gate MOS transistors in the storage cells, with hot electron injection or Fowler-Nordheim tunnel injection from the drain of the MOS transistor to the floating gate accomplishing the programming of a cell.
All of these devices, possibly with the exception of the EEPROMs, are classified as read-only memories, since the writing of data, or programming, is not easily accomplished with the memory installed in the system. In the case of a fusable link PROM, a programmed bit cannot be rewritten once programmed, while EPROMs require the exposure of the memory array to ultraviolet light for erasure of the programmed memory cells. Programmed cells of modern EEPROMs are electrically erasable by way of Fowler-Nordheim tunneling of electrons from the floating gate to the source of the transistor, requiring a tunnel oxide under a portion of the floating gate, such tunnel oxide being thinner than the gate oxide under the floating gate at the transistor channel. Such erasure requires the application of the erase voltage for a long period of time, relative to a read cycle time, limiting the use of the EEPROM as a read/write memory. In addition, programming of the PROM, EPROM and EEPROM devices, and also erasure for EEPROM devices, requires the application of relatively high voltages to the memory cells of the devices for long periods of time relative to a read cycle, making post-installation writing of data cumbersome. The high voltages further also stress the memory cell structures, limiting the number of erase and write cycles to the memories, and further limiting the applicability of these non-volatile memories as read/write memories.
As described in "A Ferroelectric Nonvolatile Memory", by Eaton et al., Digest of Technical Papers, 1988 IEEE International Solid-State Circuits Conference (IEEE, 1988), pp. 130-131, ferroelectric dielectric materials have recently been found which have certain applications for semiconductor non-volatile memories. Examples of such ferroelectric materials include modified lead titanates, such as lead zirconium titanate (PZT) as described in the Eaton et al. paper. Ferroelectric materials are polarizable by an applied electric field of sufficient strength, and have a hysteresis loop in the polarization versus electric field (P-E) characteristic. If used as the the dielectric in a parallel plate capacitor, the ferroelectric material provides a capacitance which is non-linear relative to voltage, and which has a hysteresis loop in the charge-voltage characteristic due to the hysteresis in the P-E characteristic. The "ferroelectric" name for such materials arises not from the presence of iron in the material, but from the hysteretic charge-voltage characteristic, resembling the B-H characteristic for ferromagnetic material.
Referring to FIG. 1, a charge-voltage diagram is shown for a capacitor having a PZT dielectric. It should be noted that the voltage of the horizontal axis of FIG. 1 is the relative voltage between two plates of the capacitor. The vertical axis is the charge stored on the capacitor, with the origin of FIG. 1 is at the center of the diagram. Beginning with the initial condition for the capacitor of a negative polarity voltage across the capacitor plates, shown at voltage V.sub.LO on FIG. 1, the polarization charge stored by the capacitor is negative in polarity. At this point, the PZT material is polarized into a first state, and the charge-voltage characteristic follows the right-hand curve in FIG. 1 as the voltage increases from V.sub.LO. The portion of the curve, from voltage limits A to B of FIG. 1, corresponds to a high dielectric constant capacitor, as the charge stored i proportional to the applied voltage in this region. Following the curve of FIG. 1 to the point at which the voltage across the capacitor plates reaches a high voltage, shown at voltage V.sub.HI on FIG. 1, a polarization charge of positive polarity is stored by the capacitor. The capacitor is polarized at this point into a second state. From this point, as the applied voltage decreases the charge-voltage characteristic follows the left-hand curve, again having an effective high dielectric constant for voltages between limits C and D of FIG. 1.
The high dielectric constant portion of the two curves for the two polarized states occur at different voltages from one another. For a voltage in the range of A to B of FIG. 1, a capacitor having this material for a dielectric will act as a good capacitor if in the first state (corresponding to the right-hand curve), but will act as a poor capacitor in the second state (corresponding to the left-hand curve). Similar behavior for the opposite states occurs for an applied voltage within the limits of C to D. Depending upon the characteristics of the capacitor, for some ferroelectric capacitors voltage limit A be at a lower voltage than voltage limit D, so that a range of applied voltages may exist where both states have a high dielectric constant. In addition, for some ferroelectric capacitors a built-in bias may be present so that the characteristic shown in FIG. 1 may not be symmetric relative to the zero volt condition. So long as the applied voltage across the plates within the range shown between points C and B of FIG. 1, the capacitor will remain in its previously programmed state. The above-referenced paper indicates that the switching voltage, i.e., the voltage range approximately between points D and A of FIG. 1, is on the order of 2 volts; the voltage required to polarize the capacitor is on the order of 6 to 7 volts in either polarity.
The polarization of the ferroelectric dielectric material establishes an electric field analogous to the storage of charge across the capacitor plates, even if no such charge is actually stored on the plates. As mentioned in the above-referenced paper, this behavior of the dielectric can be used as the capacitor in a single-transistor dynamic random access memory (DRAM) cell, or may be coupled to cross-coupled inverters in a static RAM cell. Non-volatility in either case results from either positively or negatively polarizing the plate by applying either a negative or positive voltage across the plates.
It is an object of this invention to provide a electrically programmable and electrically erasable floating-gate memory cell using a ferroelectric dielectric between the floating and control gates of the cell.
It is a further object of this invention to provide such a memory cell which utilizes standard MOS transistor construction under the floating gate.
It is a further object of this invention to provide such a memory cell which can utilize the same dielectric layer under the floating gate as used for MOS transistors in the same circuit.
It is a further object of this invention to provide such a memory cell which can be programmed and erased with voltages on the order of the power supply to the memory device.
Other objects and advantages will become apparent to those of ordinary skill in the art having reference to the instant specification in conjunction with the drawings.